There is increasing evidence (e.g. here) that Homo sapiens interbred with contemporary homonins such as Homo neanderthalensis and the "Denisovans". Although these homonins do show differences in their mtDNA, since a species is generally defined as a group of organisms capable of interbreeding and producing viable offspring, should we not regard humans, neanderthals and denisovans as the same species and name them as such?